1947 U.K. River Floods: 60-Year Retrospective RMS Special Report I NTRODUCTION In the second half of March 1947, the most catastrophic river floods for at least 200 years occurred in the United Kingdom. The flooding, which inundated nearly all the main rivers in the South, Midlands, and the Northeast of England, was notable for its origins, regional extent, and duration. Impacting thirty out of forty English counties over a two week period, around 700,000 acres of land were underwater. Tens of thousands of people were temporarily displaced from their homes, and thousands of acres of crops were lost. The flooding was triggered by the rapid thaw of deep snow that covered much of the country after one of the coldest and snowiest winters on record. The thaw was triggered by the arrival of a succession of southwesterly depressions, each bringing significant additional rainfall. During this period, flooding was not only widespread in the U.K., but also extended across Europe. In Spain, Germany, Poland and the Czech Republic, rivers across the continent burst their banks as meltwater and heavy rainfall combined. As the event occurred before the era of flood insurance, and in the immediate aftermath of the Second World War to a backdrop of rationing and significant post-war deprivation, the overall economic impact of the catastrophe was not fully assessed at the time as impacts were considered local rather than national. Sixty years after the event, this retrospective reviews the events of March 1947 and assesses the economic and insurance costs that would be associated with a repeat of the same event in 2007. Figure 1: Flooding of Albert Street in Windsor, one of many towns impacted by the 1947 river floods (Courtesy of P. Wells from the Royal Windsor website: http://www.thamesweb.co.uk/windsor/windsorhistory/floods47.html) 1 1947 U.K. River Floods Copyright ©2007 Risk Management Solutions, Inc. All Rights Reserved. 60-Year Retrospective T HE 1947 U.K. R IVER F LOODS The snow thaw began on March 9, 1947 as Britain started to recover from an extreme winter with heavy snowfall and persistent freezing temperatures. As milder weather finally arrived, heavy rain began to fall, causing snow across the country to melt rapidly. After weeks of frost, the ground was frozen hard and unable to soak up any moisture forcing the meltwater and rain to drain off into rivers. From March 11 through March 24, multiple river systems flooded throughout England and the eastern edge of Wales. Figure 2 shows the progression of the 1947 floods, highlighting the major rivers that flooded over the course of the event. Figure 2: Map of major rivers that flooded during the 1947 event with date of first flooding Flooding began across the South of England, from Somerset to Kent, as rivers such as the Avon in Somerset and Wiltshire, and the Medway and Great Stour in Kent overflowed. Parts of west and northeast London were inundated on March 14 and 15 as the River Thames and other surrounding rivers overflowed. Over 1,600 homes flooded at Caversham near Reading, and flood levels peaked in the Lea Valley to the east of London on March 16. The flooding of the filter beds along the River Lea to the northeast of London meant that water supplies had to be cut off to more than a million people living in east and northeast London. 2 1947 U.K. River Floods Copyright ©2007 Risk Management Solutions, Inc. All Rights Reserved. 60-Year Retrospective As a large Atlantic depression tracked across England bringing warmer temperatures, flooding extended to rivers further north. In East Anglia, the strong southwesterly winds associated with the storm of March 16 caused waves to pound the dykes that breached and flooded large areas of the low-lying Fenland. Water overflowed the channel of the River Trent on March 17, causing severe flooding in Nottingham with over 3,000 homes and 85 factories under water (Figure 3). A further 4,000 properties were reported flooded southwest of Nottingham in Long Eaton and Beeston, while more than 2,000 homes flooded in West Bridgford. Flooding peaked on March 18 and 19 when there was major flooding to the north in Yorkshire, along with continued peak flooding in parts of the Midlands and Southeast. In the Midlands, rivers flooded in Herefordshire and Gloucestershire as meltwater from the Welsh mountains ran into the Wye and Severn river basins. There were reports in Worcester of the River Severn rising 3 m (10 ft) in 24 hours. Closer to London, 1,400 homes in Maidenhead, 3,000 homes in Oxford, and 1,350 homes in Windsor were flooded. Figure 3: Flooding of Wilford Lane in Wilford, Nottingham as the River Trent burst its banks (Courtesy of Nottinghamshire Local Studies and www.picturethepast.org.uk) As river levels in the South and West of England started to recede after March 20, water levels were still rising in the Northeast, with widespread flooding extending to the rivers Wharfe, Derwent, Ouse and Aire. Almost the whole of the town of Selby, south of York, was inundated with over 70% of all properties flooded. The Lower Trent Valley was also inundated as the flood wave reached the tidal part of the river where it was impeded by a spring tide. Finally, on March 24, the River Ouse in Yorkshire peaked as the waters continued to recede in the southern part of the country. 3 1947 U.K. River Floods Copyright ©2007 Risk Management Solutions, Inc. All Rights Reserved. 60-Year Retrospective C AUSE OF F LOODING The meteorological conditions at the beginning of 1947 in England and Wales were predeterminants of the widespread and severe flooding in March. Starting in January 1947, unprecedented ‘continental’ winter weather conditions more familiar in Scandinavia or Eastern Europe hit the U.K., uninterrupted by any spells of warmer Atlantic origin conditions. Not only were these months extremely cold, but they included high levels of precipitation in the form of rain, snow and sleet. The mean maximum temperature during February 1947 in England was -0.7ºC with the mean minimum of -3.8ºC, as compared to the climatological average of 6.9ºC and 1.0ºC, respectively. Temperatures as low as -21ºC were recorded at Woburn in Bedfordshire and over 2 million sheep and lambs and 30,000 cattle perished during this freezing period. While cold winters had been known in the U.K. through the 17th and 18th centuries, cold weather was usually accompanied by reduced precipitation. However, from January to March 1947, the average precipitation for the country was 31.1 cm (12.2 in), nearly 150% of the long-term average. Snow Accumulation February 1947 was an extremely cold month, characterized by heavy snowfall in the Eastern and Midland districts of England (Figure 4). Snow lay over much of the country throughout February and early March. Around 30 cm (12 in) of ‘level’ snow persisted across areas of Durham, Yorkshire, Lincolnshire, and the East Midlands during this period. In some areas, snow lay on the ground in excess of 30 days. Persistent easterly winds, which were strong at times, led to significant snow drifts. The cold weather continued into March. On March 4, a deep depression moved through the English Channel, triggering more heavy snowfall over most of England and Wales with further severe drifting (Figure 5). During these wintry months, the snow reached its deepest level on March 6 in the amount of 42 cm (16.5 in), while the heaviest snowfall on a single day was 17 cm (6.7 in) on March 12, during a final blizzard in the Midlands. Figure 4: Deep snow along the road between Batham Gate and Peak Dale, southwest of Sheffield (Courtesy of Derbyshire Local Studies and www.picturethepast.org.uk) 4 1947 U.K. River Floods Copyright ©2007 Risk Management Solutions, Inc. All Rights Reserved. 60-Year Retrospective Figure 5: Deep snow drifts in Wales in early March 1947 (Courtesy of Carolyn Ellis) The Great Thaw A storm crossing southern England on March 10 caused more snowfall, turning to rain on March 11. As the low pressure system moved east, it allowed relatively warm, moist air from the southwest to flow across the region and cause the snow to thaw. A daily weather report from the Meteorological Office on March 12, 1947 suggests that snow cover on this day in many southern parts of the country had reached its lowest levels since January. As the snow cover continued to diminish rapidly, meteorological stations started to report ground flooding in the South and West of England. Persistent Heavy Rain Further rain was generated on the evening and night of March 16, as a storm with widespread, severe gales swept across the U.K. The windstorm had an ENE track that ran from the southern coast of Ireland across to the Humber estuary in Northeast England, with a pressure nadir of 972 mb. The storm had a bent back occlusion with very high windspeeds at the southern tip of the scorpion's tail front (as is typical of a 'sting jet'), reaching a 44 m/s (98 mph) peak gust at Mildenhall Cambridgeshire, while winds across South Wales and the Midlands were generally in the range of 31 to 36 m/s (70 to 80 mph). Across a swath from Bath to Nottingham, there were widespread reports of roads blocked by fallen trees and damage to buildings, including in London. Severe weather conditions persisted until March 24. The excessive rainfall, combining with milder temperatures, contributed to rapid snow melt across the country.